Semiconductor gas sensors are used to detect the presence of a particular gas or gasses in an environment to which the sensor is exposed. A common type of gas sensor is a metal oxide semiconductor (MOS) gas sensor, which is also referred to as a “thick-film” gas sensor. FIG. 1 shows a prior art MOS gas sensor 10 including a substrate 14, a gas-sensitive portion 18 in electrical communication with electrodes 22, and a heating element 26. The gas-sensitive portion 18 is a thick-film that is configured to undergo a change in ionic conduction, electronic conduction, and/or optical transmittance in the presence of a target gas. The change of the gas-sensitive portion 18 is detected by an external read-out circuit (not shown) that is electrically connected to the electrodes 22. Typically, the change is exhibited as a change in the electrical resistance of the gas-sensitive portion 18, as measured by the read-out circuit. The heating element 26 is activated to heat the gas-sensitive portion 18 to a temperature that is suitable for detecting the target gas.
The ionic and/or electrical change in the gas-sensitive portion 18 in the presence of the target gas is a catalytic reaction. The surface of the gas sensitive portion 18 typically includes adsorbed molecules, which participate in the gas sensing process. For example, the surface of an n-type gas sensitive portion 18 typically includes adsorbed oxygen molecules. Each adsorbed oxygen molecule results in an electron hole (h*) that contributes to the electrical conduction of the gas sensitive portion 18 and tends to reduce the electrical resistance of the gas sensitive portion 18 according to the following formula:½O2 (g)O−(ads)+h*. When the n-type gas sensitive portion 18 is in contact with a molecule of the target gas, carbon monoxide (CO) for example, the gas sensitive portion undergoes a local change in chemical potential that leads to desorption of the adsorbed oxygen molecule according to the following formula:CO (g)+O−(ads)+h*→CO2 (g)Along with desorption of the oxygen molecule, an electron is recombined with the hole (h*) to eliminate the hole, thereby increasing the electrical resistance of the gas sensitive portion, as sensed by the external read-out circuit. A corresponding relationship exists for a p-type gas sensitive portion 18, except that each adsorbed oxygen molecule gives rise to an electron instead of a hole (h*). Therefore, the underlying principle of operation of a MOS gas sensor includes exposing the gas sensitive portion 18 to a target gas, which results in a change in chemical potential of the gas sensitive portion. The change in chemical potential results in ionic/electronic exchange, which causes modulation of the electrical resistance of the gas sensitive portion 18. The modulation of the electrical resistance is sensed using the external read-out circuit and represents the presence, concentration, and/or the absence of the target gas.
The change in optical transmittance of a thick film gas sensor in the presence of the target gas is also a catalytic reaction. Optical thick film MOS gas sensors are found, for example, in carbon monoxide detectors and typically include an optical gas sensor and a corresponding read out circuit. The gas sensor includes a gas sensitive portion formed from a thick film of tin dioxide and nickel oxide, for example, that has been heat treated (annealed) at approximately 500° C. A heater circuit heats the thick film to an operating temperature, and the read out circuit monitors the optical transmittance of the heated thick film, which varies based on the concentration of carbon monoxide in the environment to which the detector is exposed
In some instances it is desirable to “enhance” or “activate” the catalytic nature of the gas sensitive portion of a thick film MOS gas sensor. As an example, when either tin dioxide (SnO2) or lanthanum oxide (La2O3) is used as the exclusive gas sensing material of a sensor device, the sensor device is unable to sense the presence of carbon dioxide (CO2); however, when the two materials are layered upon each other, the junctions/boundaries of the materials are catalytically activated and become sensitive to carbon dioxide through a process referred to as mutual induction. Thus, tin dioxide and lanthanum oxide are referred to as being mutually catalytic materials.
As described above, MOS gas sensors are useful for sensing a target gas; however, fabricating a thick film MOS gas sensor can be difficult and time consuming, especially when the gas sensitive portion includes multiple layers of mutually catalytic materials. For example, to form the gas sensitive portion 18, powered tin dioxide and an organic binder are made into a paste that is screen printed onto the substrate 14. Then the substrate 14 and the paste are annealed at 600° C. for one hour. Next, lanthanum chloride (LaCl3) and a solvent are applied to the annealed paste and are Joule heated (using the heating element 26, for example) to 120° C. for five minutes in order to evaporate the solvent. Thereafter, the Joule heated structure is annealed from 400° C. to 1200° C. in steps of 200° C. for five minutes to form the gas sensitive portion 18. The above described technique typically works well in a research environment, but is unsuitable for implementation in a mass-production line.
Therefore, for at least some of the above-described reasons, it is desirable to improve the structure and the process for fabricating semiconductor gas sensor devices.